The present invention relates generally to a paperless gypsum/fiber board with improved surface characteristics, to a process for making such a gypsum/fiber board, and to a primer for use on such gypsum/fiber board to provide improved surface characteristics. More particularly, the present invention relates to a primer to improve the surface characteristics of fiber-reinforced gypsum panels.
Conventional gypsum wallboard or panel is typically manufactured from a plaster slurry wherein a wet slurry of calcium sulfate hemihydrate, generally referred to as calcined gypsum, is placed between two layers of paper and the slurry is allowed a certain amount of time to set. The set gypsum is a hard and rigid product obtained when the calcined gypsum reacts with water to form calcium sulfate dihydrate. Calcined gypsum is either calcium sulfate hemihydrate (CaSO4.{fraction (1/2+L )} H2O) or calcium sulfate anhydrite (CaSO4). When calcium sulfate dihydrate is heated sufficiently, in a process called calcining, the water of hydration is driven off and there can be formed either calcium sulfate hemihydrate, calcium sulfate anhydrite, or both, depending on the temperature and duration of exposure. When water is added to the calcined gypsum to cause the gypsum to set, in essence, the calcined gypsum reacts with water, and the calcined gypsum is rehydrated.
In typical gypsum wallboard, the two layers of paper contain the slurry and provide the strength required in installation and use. The wallboard is cut into discrete lengths to accommodate subsequent handling and then dried in heated dryers until the board is completely dry. The bending strength of the wallboard depends largely on the tensile strength of the paper. The gypsum serves as the core and accounts for fire resistance and moisture absorbing and moisture releasing activities. The paper determines the nature of the joint filler system and the surface treatment that may be used on the board.
Although paper-covered wallboard has many uses and has been a popular building material for many years, the prior art has recognized that for certain applications it would be advantageous to provide gypsum panel that did not rely on paper surface sheets for strength and other properties. Several prior art fiber-reinforced gypsum panels are as follows:
U.S. Pat. No. 5,320,677 to Baig, which is incorporated by reference herein in its entirety, describes a composite product and a process for producing the product in which a dilute slurry of gypsum particles and cellulosic fibers are heated under pressure to convert the gypsum to calcium sulfate alpha hemihydrate. The cellulosic fibers have pores or voids on the surface and the alpha hemihydrate crystals form within, on and around the voids and pores of the cellulosic fibers. The heated slurry is then dewatered to form a mat, preferably using equipment similar to paper making equipment, and before the slurry cools enough to rehydrate the hemihydrate to gypsum, the mat is pressed into a board of the desired configuration. The pressed mat is cooled and the hemihydrate rehydrates to gypsum to form a dimensionally stable, strong and useful building board. The board is thereafter trimmed and dried. The process described in Patent No. 5,320,677 is distinguishable from the earlier processes in that the calcination of the gypsum takes place in the presence of the cellulosic fibers, while the gypsum is in the form of a dilute slurry, so that the slurry wets out the cellulosic fibers, carrying dissolved gypsum into the voids of the fibers, and the calcining forms acicular calcium sulfate alpha-hemihydrate crystals in situ in and about the voids.
U.S. Pat. No. 5,135,805 to Sellers et al, describes a water resistant gypsum product that may be a xe2x80x9cfacelessxe2x80x9d product, i.e. it may not include a facing sheet of paper, fiberglass mat or similar material. The gypsum products described by U.S. Pat. No. 5,135,805 typically contain reinforcing fibers, for example, cellulosic fibers, such as wood or paper fibers, glass fibers or other mineral fibers and polypropylene or other synthetic resinous fibers. The reinforcing fibers can be about to about 20 wt. % of the dry composition from which the set gypsum product is made. The density of such a product is typically within the range of about 50 to about 80 pounds per cubic foot.
U.S. Pat. No. 5,342,566 to Schafer et al, which is incorporated by reference herein in its entirety, refers to a method of producing fiber gypsum board comprising the steps of mixing in a preliminary mixing step predetermined amounts of fibers and water respectively, to form a mixture of wetted, loose fibers; mixing in a mixing step the wetted fibers with a predetermined amount of dry calcined gypsum; premixing an accelerator with one of the components of dry calcined gypsum, fiber and water; promptly laying the mixed composition into a mat; immediately degassing the mat in a first compression step, adding a predetermined amount of water onto the resultant mat; and immediately compressing the mat to form a board composed of bonded fibers and gypsum. This process produces a homogeneous board which is preferably a gypsum board reinforced by fiber, such as paper fiber, wherein several layers of board forming materials are placed on each other before the board is fully formed, pressed, and dried and wherein each of the layers is identical in composition.
U.S. Pat. No. 5,637,362 to Chase et al, which is incorporated by reference herein in its entirety, describes a thin, sealant-coated, fiber-reinforced gypsum panel. This patent describes a wax-free, water-resistant xe2x80x9csealantxe2x80x9d which is applied to the board after the board has been dried. The sealant, which preferably is a siliconate mixture, reduces the absorption of water, glue or adhesives into the panel.
Carbo et al Provisional Application Serial No. 60/073,503, which is incorporated by reference herein in its entirety, describes a paperless gypsum/fiber board with improved surface characteristics, a process for making such a gypsum/fiber board, and a primer for use on such gypsum/fiber board to provide improved surface characteristics.
The term xe2x80x9cpaperlessxe2x80x9d gypsum/fiber board, as used herein, is intended to distinguish the fiber-reinforced gypsum panels to which the present invention applies from conventional prior art gypsum panels, which are referred to as xe2x80x9cwall boardxe2x80x9d or xe2x80x9cdry wallxe2x80x9d which have at least one surface comprised of paper, including xe2x80x9cwall boardxe2x80x9d or xe2x80x9cdry wallxe2x80x9d having some additional form of fiber-reinforcement in the core.
The prior art paperless fiber-reinforced gypsum panels have surface characteristics which differ from the conventional paper-covered wallboard and the difference in the surface characteristics creates an array of problems for the tradesmen who use or install such panels. For example, some of the prior art fiber reinforced gypsum panels tend to absorb water quickly. As a consequence, joint compound applied to such panels does not have sufficient xe2x80x9cworking timexe2x80x9d, i.e. the joint compound dries out before it can be finished properly. Similar problems are encountered in the application of adhesives and paints to the fiber reinforced gypsum panels. Another problem occurs when paints and coatings applied to the fiber reinforced gypsum panels for decorative purposes do not consistently flow over and wet the surface of the panels. This causes major problems in decorating the panels.
However, some coatings or sealants, such as the siliconate sealant described in U.S. Pat. No. 5,632,362 to Chase et al, produce a seal that is too complete, that is they seal the surface so effectively that the xe2x80x9csuctionxe2x80x9d of the surface of the board is reduced which results in deteriorated joint compound performance, and increased drying time for paint.
It is the objective of the present invention to provide a primer system that overcomes these problems in paperless fiber-reinforced gypsum panels.
It is a further objective of the present invention to provide a primer system for paperless fiber-reinforced gypsum panels having improved surface characteristic.
It is yet a further objective of the present invention to provide a primer system for paperless fiber-reinforced gypsum panels having improved paint and joint compound performance.
The present invention relates generally to a paperless gypsum/fiber board with improved surface characteristics, to a process for making such a gypsum/fiber board, and to a primer for use on such gypsum/fiber board to provide improved surface characteristics. More particularly, the present invention relates to a primer to improve the surface characteristics of paperless fiber-reinforced gypsum board. Still more particularly, the present invention relates to a primer to improve the paint and joint compound performance characteristics of paperless fiber-reinforced gypsum board. The primer is designed to be applied to a fiber-reinforced gypsum board of the type described in the foregoing prior art or any of the prior art paperless fiber-reinforced gypsum board.
The primer of the present invention is a water-based system that must be heated after application to effect a cure. The primer is based on a latex, that is an aqueous emulsion of a film forming polymeric material having a glass transition temperature in the range of from 10 to 40xc2x0 C. and preferably in the range of 12 to 20xc2x0 C. The polymeric material is selected from acrylic polymers or copolymers and mixtures of acrylic polymers or copolymers with other thermoplastic polymers. The primer may be pigmented to give the primer a color and it may contain a filler such as limestone or silica, depending upon the end-use of the panels.
The primer is preferably applied to the surface of the board during the manufacturing process, before the board is dried, in order to cure the primer on the surface of the board as the board is dried.
The primer provides the following improvements to the surface of the board:
1. Provides a uniform, smooth surface for the board;
2. Provides good adhesion for paint, mortar and adhesives which are subsequently applied to the board;
3. Provides water resistance which protects the adhesion of paint, mortar and adhesives to the board even though the board comes in contact with water;
4. Provides uniform xe2x80x9csuctionxe2x80x9d and uniform xe2x80x9cabsorptionxe2x80x9d of water from paint, adhesives, and joint compounds, so that the paint, adhesives, and joint compounds have adequate xe2x80x9cworking timexe2x80x9d; and
5. Provides improved scuff resistance to the panels both before and after decoration.
The present invention relates generally to a paperless gypsum/fiber board with improved surface characteristics, to a process for making such a gypsum/fiber board, and to a primer for use on such gypsum/fiber board to provide improved surface characteristics. The primer of the present invention may be used with any of the prior art fiber-reinforced gypsum panels or with any of the newly formulated fiber reinforced gypsum panels as described in Provisional Application 60/073,503.
The present invention contemplates the formation of fiber-reinforced gypsum board having a homogeneous structure throughout, as well as composite boards having two or more distinct layers. It is contemplated that the primer of the present invention will be used on a board having at least one surface comprised of fully hydrated gypsum and reinforcing fiber.
The primer of the present invention is a water-based system that must be heated after application to effect a cure and to achieve the improvements in the surface characteristics of paperless gypsum/fiber board described above. The primer is based on a latex, that is an aqueous emulsion, of a film forming polymeric material having a glass transition temperature in the range of from 10 to 40xc2x0 C. and preferably in the range of 12 to 20xc2x0 C. Polymeric materials having a Tg in the 12 to 20xc2x0 C. range are preferred because less heat is required to form a film on the board surface.
The polymeric material is selected from film forming acrylic polymers or copolymers and mixtures of acrylic polymers or copolymers with other thermoplastic polymers. As used herein the term xe2x80x9cpolymersxe2x80x9d is intended to include both homopolymers and copolymers, and the term xe2x80x9ccopolymersxe2x80x9d is intended to include polymers made from two or more different monomers. The acrylics are preferred because they don""t swell in the presence of alkalis. The polyvinyl acetate/acrylate copolymers may be used to soften the acrylic polymers. The acrylics may be copolymerized with vinyls and/or styrene to provide polymers with appropriate glass transition temperatures.
The present invention contemplates the use of latexes that include acrylics, vinyl acrylics, polyvinylacetate/acrylics, styrene/acrylics, ethylene/vinylchlorides and ethylene/vinylchloride/vinylacetates, provided they are film formers and have glass transition temperatures within the 12 to 40xc2x0 C. range.
Among the commercially available acrylic emulsions that may be used are POLYCO 3103, an aqueous emulsion containing about 50% by weight of a vinyl acetate/acrylic copolymer having a glass transition temperature of 13xc2x0 C., which is sold by Rohm and Haas Company. Other commercially available emulsions that may be used are listed in Table 1, below.
High Tg polymers may be blended with low Tg polymers in order to improve film formation using the high Tg polymers. Plasticizers and softening agents such as esters and alcohols may be added to the polymeric material to aid coalescence of the polymeric material and thus lower the effective glass transition temperature of the polymer.
The primer may be pigmented to give the primer a color and it may contain a filler such as limestone or silica. Primers containing pigments and/or fillers tend to minimize board-to-board sticking. A variety of conventional additives, such as defoamers or biocides, may also be included in the primers.
It has been found necessary to apply a volume of primer sufficient to completely wet the surface of the board in order to obtain a uniform coating of the primer on the board. It is preferred to use from about at least 14 grams and up to 20 grams of a clear aqueous primer (e.g. composition E of Table 2) per square foot of board to obtain the best improvement in surface characteristics. The most preferred application range for clear aqueous primer is about 18 grams of primer per square foot of board. The preferred application range for a pigmented primer (e.g. composition B of Table 2) is from about at least 7.5 grams and up to about 15 grams per square foot of board.
In order to achieve the best surface characteristics on board designed for interior use, i.e. board designed to compete with conventional wallboard, about 0.50 to about 1.00 grams of polymer solids per square foot of board should be used. Using less than 0.50 grams of polymer solids per square foot of board sometimes results in an incomplete film on the surface, which may reduce the xe2x80x9copen timexe2x80x9d slightly when coated with a semi-gloss latex, as shown in Table 7. However using less than 0.50 grams of polymer solids per square foot of board provides some improved surface characteristics. Using more than 1.0 grams of polymer solids per square foot tends to produce a seal that is too complete, which reduces the xe2x80x9csuctionxe2x80x9d of the surface of the board, deteriorates joint compound performance, and increases drying time of paint. The surface characteristics of underlayment boards may be improved by applying pigmented primers at about 0.35 to about 0.70 grams of polymer solids per square foot of board.
The primer of the present invention may be applied to the fiber-reinforced gypsum board using a variety of techniques including spraying, roll coating or curtain coating. The Example 1, which follows, illustrates a flood coating technique to apply the primer.
The primer of the present invention may be applied to the fiber-reinforced gypsum board at any desired point in the manufacture of the board. Because the primer must be heated after application it is preferred to apply the primer to the board prior to the final drying of the board. However, the primer may be applied to the board after the board is completely dried, provided the board is then heated again under conditions sufficient to cure the primer.
The primer of the present invention may be dried on the fiber-reinforced gypsum board under a variety of conditions. Generally, the board must be heated under conditions that remove all of the water from the primer coating and heat the primer sufficiently to coalesce the polymeric particles and seal the surface of the board. Generally, applying air heated to 100 to 500xc2x0 F. to the coated board for 1 to 10 minutes in a dryer is adequate to cure the primer.
Table 2, below shows the useful ranges of materials, by weight, which may be used in the preferred type of primers, along with formulations for 4 specific, useful primers.
The latex used in composition A was POLYCO 3103, an aqueous emulsion containing about 50% by weight of a vinyl acetate/acrylic copolymer having a glass transition temperature of 13xc2x0 C., and is sold by Rohm and Haas Company. Composition A is a useful primer for interior wallboard.
The latex used in composition B POLYCO 3103, an aqueous emulsion containing about 50% by weight of a vinyl acetate/acrylic copolymer having a glass transition temperature of 13xc2x0 C., and is sold by Rohm and Haas Company. Composition B has been used as a primer for interior panels and as a primer for floor underlayment boards. Preferably, the primer for underlayment boards is pigmented and is applied to both surfaces of the underlayment board.
The latex used in composition C was EI 8764, an aqueous emulsion containing about 50% by weight of an all acrylic copolymer having a glass transition temperature of 10xc2x0 C., and is sold by Rohm and Haas Company. Composition C is a useful primer for ceramic tile backing because it provides better water resistance.
The latex used in compositions D, E and F was POLYCO 3103, an aqueous emulsion containing about 50% by weight of a vinyl acetate/acrylic copolymer having a glass transition temperature of 13xc2x0 C., and is sold by Rohm and Haas Company.
The following example will serve to illustrate the manufacture of a gypsum/fiber board product that has improved surface characteristics by the use of the primer of the present invention. However, it should be understood that this example is set forth for illustrative purposes and that many other gypsum fiber products are within the scope of the present invention.